1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming system.
2. Description of the Related Art
This type of sheet conveying apparatus is sometimes used at the time of performing post-processing of a sheet discharged from an image forming apparatus such as a printer, a copying machine, or a printing machine through a plurality of sheet post-processing devices. As the post-processing device, for example, there can be mentioned a punching device that punches a sheet, a sheet binding device that binds a bundle of sheets by a stapler or the like, and a sorter/ejector that sorts sheets formed with images to a plurality of paper discharge trays and discharges the sheets. Such a sheet post-processing device is often unitized individually and is selectively installed in the image forming apparatus.
In an image forming system in which a plurality of sheet post-processing devices are installed, sheets are conveyed along a predetermined sheet conveyance path while being transferred between the sheet post-processing devices. In such an image forming system, sheets may be conveyed while a conveying force is applied simultaneously from two sheet post-processing devices. In this case, when the sheet conveyance speed of the two sheet post-processing devices is different from each other, sheets may be slackened or pulled depending on the speed difference, thereby causing a problem of damaging the sheets or jamming.
In order to solve the problem, Japanese Patent No. 4486480 discloses a sheet conveying apparatus that when the conveyance speed of two units (sheet post-processing devices) that transfer sheets is different from each other, synchronizes a change timing of the conveyance speed. Specifically, at the time of accelerating or decelerating the speed or start-stop between the two units, the sheet conveying apparatus synchronizes the two units so that the accelerated or decelerated rate and the accelerated or decelerated distance of the two units becomes equivalent. With this configuration, occurrence of excessive slackening or pulling is suppressed in the sheets conveyed over the two units.
In the sheet conveying apparatus described in Japanese Patent No. 4486480, sheet conveying means such as a conveyance roller pair that applies a conveying force to the sheet in each unit is driven by a stepping motor. In this manner, when a drive source of the sheet conveying means of each unit is a synchronous motor such as the stepping motor, a highly accurate rotation position (sheet position) control during acceleration or deceleration can be executed. Accordingly, a control such that excessive slackening or pulling does not occur in the sheet conveyed over the sheet conveying means of the both units is facilitated during acceleration or deceleration.
However, when an asynchronous motor such as a direct-current motor (DC motor), for which a highly accurate rotation position control is difficult, is adopted for one of the drive sources, it is difficult to control so that excessive slackening or pulling does not occur in the sheet conveyed over the sheet conveying means of the both units in an acceleration period or a deceleration period. More specifically, when the drive sources are both the asynchronous motor, the both motors have the same acceleration characteristics and deceleration characteristics. Therefore, by standardizing a drive start timing and a drive stop timing of the both motors, excessive slackening or pulling of sheets in the acceleration period and the deceleration period can be suppressed without executing any rotation position control. However, when one of the drive sources is the asynchronous motor and the other one is the synchronous motor, the acceleration characteristics and the deceleration characteristics of the both motors are considerably different. Therefore, when the standardized drive start timing or drive stop timing is used, excessive slackening or pulling of sheets occurs in the acceleration period and the deceleration period.
At this time, such a method can be considered that a rotation position of the synchronous motor in the acceleration period and the deceleration period is controlled according to the acceleration characteristics and the deceleration characteristics of the asynchronous motor, so that excessive slackening or pulling of sheets does not occur in the acceleration period and the deceleration period. Generally, however, even if the synchronous motor is accelerated so as to reach a target speed as soon as possible, the magnitude of acceleration of the synchronous motor is far less than that of the asynchronous motor. The same applies to the case of deceleration. Therefore, according to the method of controlling the rotation position of the synchronous motor, it is difficult to suppress occurrence of excessive slackening or pulling of sheets in the acceleration period and the deceleration period.
Therefore, it is desirable to provide a sheet conveying apparatus that can suppress occurrence of excessive slackening or pulling of a sheet conveyed over two sheet conveying units that respectively adopt a synchronous motor and an asynchronous motor, and an image forming system including the sheet conveying apparatus.